Recently, for drive motors used in general industrial machinery, machine tools and the like, motors having a very high rotation speed have been emerged. In particular, motors for driving spindles of machine tools have a strong tendency of increase in speed in accordance with increasing speed of spindles of machine tools.
In a case of a medium/high-speed spindle, for example, if a revolution number of the spindle is about 10000 min−1 or less, a drive motor having a highest rotation speed of about 5000 to 8000 min−1 may be used, and gears or belts may be used to increase the speed. However, in a case of a high-speed spindle, for example, if a revolution number of the spindle is about 15000 to 20000 min−1, a speed-up ratio by gears or belts becomes two times or more, and thus to transmit an appropriate driving force, diameters of gears or diameters of belt pulleys are increased also, thereby significantly increasing a peripheral speed or a rotation speed in such transmission portion. As a result, in the case of gear driven type, a noise in meshed portions of gears and wearing or chipping of teeth thereof are likely to occur, and in the case of belt driven type, slipping, flapping, wearing, breaking and the like of belts are likely to occur.
Due to these reasons, a method of driving a spindle is being changed from gear-driven or belt-driven methods to a direct driving method using a coupling. With a direct coupling connection driving method, a driving torque is directly transmitted to the spindle via the coupling, and therefore a load component by a driving force is not generated in a bearing that supports a drive motor. However, because a rotation speed of the drive motor becomes the same revolution number as that of the spindle, high-speed bearings have correspondingly been required.
Conventionally, for 4-pole or 2-pole general purpose motors (a revolution number of about 1500 to 3600 min−1), deep groove ball bearings (not shown) employing a press-molded iron cage 100 having spherical pockets 111 as shown in FIG. 10 have been used. More specifically, the cage 100 has a pair of annular portions 110, 110 coupled to each other in an axial direction. The annular portions 110 includes, when being coupled to each other in the axial direction, a plurality of pockets 11 for rollably retaining a plurality of balls 103, and a plurality of flat surface portions 112 each provided between adjacent pockets 111, 111. The flat surface portions 112, 112 of the pair of annular portions 110, 110 are fixed to each other by metal rivets 113.
For medium/high-speed motors (a revolution number of about 5000 to 8000 min−1), deep groove ball bearings employing a high wear resistant crown type synthetic resin cage 200 having spherical pockets 211 like the cage 100 of FIG. 10 as shown in FIG. 11 have been used. More specifically, the crown type synthetic resin cage 200 has a substantially annular base portion 213, and a plurality of pillar portions 217 protruding from an axial end face 215 of the base portion 213 and arranged at given intervals in a circumferential direction, and the opposing surfaces 219, 219 of a pair of adjacent pillar portions 217, 217 and the axial end face 215 of the base portion 213 form the pocket 211 for retaining a rolling element 220.
As for other cages, a synthetic resin cage 300 has been devised, in which, as shown in FIG. 12, hemispherical pockets 304 for retaining rolling elements (not shown) are provided on two pieces of annular portions 301, 301 at equal intervals in a circumferential direction and engaging pawls 306 and engaging holes 305 that are engageable with each other are provided on coupling portions 303 between the adjacent pockets 304 (see Patent Document 1).
Further, according to Patent Document 2, as shown in FIG. 13, a crown type cage 400 has a plurality of pockets 402 opened to both sides in a radial direction, each pocket 402 being formed such that a straight hole 405 having an peripheral wall 404 extending along a center line in the radial direction is formed, and projections 406, 406 for retaining the ball 413 is formed on an inner diameter-side edge of the straight hole 405 so as to be opposed to each other in a direction toward the center axis of the straight hole 405. This configuration is aimed to suppress vibrations of the cage in the radial direction, thereby improving abrasion and wear resistance properties and preventing generation of a cage noise during rotation.